Although magnetostriction was first discovered by Joule in 1847 in iron, high magnetostriction (almost 1%) was first discovered in the rare-earth elements dysprosium (Dy) and terbium (Tb) at cryogenic temperatures in 1963.
Both terbium and dysprosium derive their magnetism from the partially filled, outermost shell of electrons (4f). The anisotropic distribution of electrons in this shell results in highly anisotropic magnetic and magnetostrictive behaviors. This anisotropy causes the magnetostriction to change suddenly when an activation magnetic field is applied. Below this activation field, there is little magnetostriction and above it the material is saturated at the maximum magnetostriction. Terbium displays a positive anisotropy whereas dysprosium has a negative anisotropy.
To obtain a smooth magnetostriction, the two elements are alloyed together. The alloy Tb0.6Dy0.4 exhibits the highest magnetostriction (6300 ppm) but its ordering temperature is about 150K. As an actuator material, it is poor because of its limited mechanical strength.
Since 1995,the Tb1-xDyxZn alloy has emerged as the preferred magnetostrictive material for applications at temperatures below ˜150K. See U.S. Pat. No. 4,906,879.The magnetostriction of this alloy is comparable to the huge magnetostriction of the rare earth elements Tb and Dy themselves (˜0.5% at 77K) and this material is stronger than TbDy. Single crystals are required in almost all cases. For the rare earth elements, orientation is very important since the magnetization remains essentially in the basal plane for all practical magnetic fields. Crystallites oriented in directions out of the plane produce almost no magnetostriction.
A search for high magnetostriction materials at room temperature led to alloying of these rare-earth materials with transition metals such as iron (Fe), culminating in the discovery of high magnetostriction in the Laves phase compound TbFe2. Soon thereafter, dysprosium was added to this compound to reduce the anisotropy. Tb1-xDyxFe2-y (0.7≦x≦0.8, 0≦y≦0.1) represents the room temperature magnetostrictor commonly referred to as Terfenol-D. See U. S. Pat. No. 4,308,474.
Tb0.3Dy0.7Fe1.95 has been described as the optimum composition for room temperature magnetostrictive applications. According to Hathaway and Clark [MRS Bulletin, Vol. XVIII, No. 4,pp. 34–41], this compound shows the highest magnetostriction at room temperature. Measurements of magnetostriction at temperatures below room temperature indicate the magnetostriction increases for decreasing temperature to 250K and then decreases rapidly as the temperature decreases below that range, leading to the conclusion that this material is not a good magnetostrictor at cryogenic temperatures (<250K). See FIG. 1.